1. Field of the Invention
The present invention relates to a small-amplitude serial data transfer technique, and, more specifically to an automatic adjustment circuit for amplitude of differential signal of a circuit in which electric signals used for transmission and reception are differential signals.
2. Background Art
Differential signals used for small-amplitude serial data transfer have small amplitude. A level of the amplitude needs to be within a range set by a standard.
However, when variation occurs because of fluctuation in a process, a temperature, a voltage, and the like of a circuit (LSI) that generates the differential signals, in some case, the level of the amplitude is not within the standard range unless a margin is provided for the level.
In the past, in order to prevent deviation from the standard range, in some cases, there is a circuit having a function that can change differential amplitude according to setting of an internal register.
However, in such cases, in order to discriminate whether an outputted differential amplitude is appropriate, a measurement circuit/device has to be connected to the outside of the circuit (LSI) to measure an amplitude and register setting has to be performed on the basis of a result of the measurement. Therefore, time and labor are required for adjustment.
For example, a conventional squelch circuit of the past amplifies output signals of a differential pair on the basis of an input signal of the differential pair and holds a peak value of the output signal of each of the differential pair in a node, a potential of which is returned to a potential associated with a signal undetected state by a certain time constant. As such a squelch circuit, there is a squelch circuit that compares the potential of the node and a potential of a reference level (see, for example, Japanese Patent Application Laid-Open Publication No. 2002-344541).
Consequently the reception is accurately discriminated even if the input signal of the differential pair is rapid and has very small amplitude.
However, the squelch circuit of the past does not discriminate whether the outputted differential amplitude is appropriate and cannot adjust the amplitude itself of the differential signals.